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Ten years on, a rich haul of planets

TEN years ago this week, the world awoke to a discovery that changed perceptions of our place in this universe forever. On 6 October 1995, Didier Queloz and Michel Mayor of the Geneva Observatory in Switzerland went public with their discovery of a planet circling another sun-like star. It was the latest milestone on the journey Copernicus began when he concluded that Earth was not the centre of the solar system. Now even our solar system was no longer a one-off.

Some 160 “exoplanets” later, nothing quite like our own solar system has yet been found. But as the catalogue of alien worlds expands, trends are emerging&colon; some understandable, some baffling, but all challenging our preconceptions of how planets form. There are hints of Earth-like worlds out there, and new technologies for detecting them promise to make the next decade as dizzying as the one gone by. “We are very lucky that this field is so fresh and new,” Queloz says. “Any step you do, you’re finding out something new.”

The planet Queloz and Mayor found 10 years ago is nothing like Earth. It is a gas giant like Jupiter, but about half as massive. Unlike Jupiter it is perilously close to its parent star, 51 Pegasi, which it takes a mere four Earth days to orbit, and its outer layers are at a blistering 1300 kelvin.

Since then, a team led by Geoff Marcy of the University of California at Berkeley and Paul Butler at the Carnegie Institution of Washington in Washington DC has been responsible for most of the other exoplanets found so far. Astronomers have found more than a dozen other “hot Jupiters” of the kind orbiting 51 Pegasi. They orbit their stars at a tenth of Earth’s distance from the sun, and do it in less than 12 Earth days.

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Initially the hot Jupiters baffled astronomers. Planets are thought to form in the disc of dust and gas around a newborn star, and it was assumed that gas giants form in the disc’s cooler outer reaches, where an embryonic planet can trap vast amounts of gas and ice. “It is extremely difficult to form giant planets very close to stars,” says Jack Lissauer of NASA’s Ames Research Center in Moffett Field, California. And it is theoretically impossible for hot Jupiters to have formed at their present locations.

So now the consensus is that the hot Jupiters formed farther out and then migrated inwards. This could happen if material in the planet-forming disc gradually saps the planet’s angular momentum, or if the disc’s gas and dust swirls into the star, dragging the giant planet along. “According to the models, migration over large distances is possible,” Lissauer says.

Apart from some quirky planets (see “A motley bunch”), the exoplanets discovered so far are typically about twice as massive as Jupiter, take from a few Earth months to a couple of Earth years to orbit their star, and are cooler than the hot Jupiters.

Taken together, these exoplanets reveal a clear pattern&colon; most have much more elliptical or “eccentric” orbits than the planets of our solar system, which, apart from Pluto, have nearly circular orbits, with eccentricities of less than 5 per cent. The average eccentricity for the known exoplanets is about 25 per cent.

And last month, Hugh Jones of the University of Hertfordshire, UK, and member of the Anglo-Australian Planet Search team, announced the discovery of two giant planets with exaggerated eccentricities of 85 and 92 per cent at a meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS) in Cambridge, UK. These planets sweep through their solar systems like crazy gigantic comets.

Astronomers are struggling to understand these orbits. Perhaps the planets formed with nearly circular orbits, but the gravity of multiple planets within each system amplified their eccentricities. An alternative, suggested by Gennaro D’Angelo of the University of Exeter, UK, at the DPS meeting, is that a giant planet’s gravity induces instabilities in the surrounding disc, making it eccentric, and that this increases the planet’s orbital eccentricity. Simulations show that this process can boost planetary eccentricities to about 40 per cent.

But eccentricities above 80 per cent continue to defy explanation. It is very unlikely that planets can be born into such highly eccentric orbits. “I don’t think any mechanism proposed so far can explain that,” D’Angelo says. It is possible, however, that they were sent careering off course by a rare encounter with a passing star.

One particularly intriguing observation is that the likelihood of a star having detectable planets is roughly proportional to the square of the number of iron atoms it contains. This has inspired a debate over “nature versus nurture” in planetary nurseries. The “nature” side argues that the disc around a young star clumps most easily into planets if it contains plenty of heavy molecules, while the “nurturers” argue that stars with planets simply seem metal-rich because the star’s so-called convective zone (its outer layers) is polluted with planetary debris.

Lissauer favours the “nature” explanation. “The observed correlation between stellar metallicity and planetary occurrence is strong evidence that heavy elements are needed to form giant planets,” he says. This is backed up by the observation that stars with planets appear to have enhanced heavy elements even beneath the convective zone.

Another mystery concerns the densities of exoplanets. A planet’s size and density can only be measured if it passes directly in front of its star, as viewed from Earth, and only eight such “transiting” planets have been found. One of these is less massive than Jupiter but 35 per cent wider. Another has Saturn’s mass but is much smaller, unexpectedly dense, and might have a solid core of heavy elements about 70 times as massive as Earth.

As planet-hunting techniques improve, there are hints that Earth-like worlds might turn out to be common

Astronomers cannot explain the difference between these planets and our gas giants. “Maybe the formation process is different, but why?” Queloz says. “It’s very exciting. You realise that our solar system is just one example of the many ways that nature is building planets.”

So far, the vast majority of new planets have orbital periods shorter than that of Jupiter, which is to be expected. To discover a planet, it is necessary to monitor its parent star for at least one orbital period, making planets with short periods easier to spot. “There will be plenty of Jupiters – we just need time,” Queloz says.

Alien Earths remain elusive, though. The smallest exoplanet to date, found by Marcy and Butler’s team in June, is about seven times Earth’s mass. But as technology becomes more sensitive, there are hints that Earth-like worlds might turn out to be common&colon; the trend so far is that astronomers are discovering more smaller-mass planets than larger ones.

The discovery of smaller planets should fill out our picture of the systems in which giants have been found. Work by Rory Barnes and Richard Greenberg at the University of Arizona in Tucson shows that the giant planets in multiple systems, including our own, are packed in as closely as they can be. Try to pack them any closer, and mutual gravity would knock them off their orbital tightropes. Already, 18 of the systems found have two, three or four planets. “There may be a large number of smaller, currently undetectable companions packed in orbits around stars with known planets,” Barnes concluded at the DPS meeting.

The next decade could yield a rich haul, as planet-hunting techniques improve (see Graphic). Better ground-based telescope surveys should reveal true analogues of Jupiter and Saturn in long, sedate orbits. They may also turn up small, rocky planets close to their stars. “This will be our first hint of how many Earth-like planets might exist,” says Butler.

However, finding genuine Earth analogues with Earth-year orbits will probably have to await new space telescopes. In mid-2006, the French space agency CNES hopes to launch a spacecraft called COROT that will simultaneously monitor 12,000 stars. NASA aims to up the ante in 2008 with Kepler, a 95-centimetre space telescope that will monitor some 100,000 stars at once. These could discover several hundred planets the size of Earth or bigger.

Later missions include NASA’s SIM PlanetQuest, which plans a highly accurate “deep search” for terrestrial planets around 250 nearby stars, and the Terrestrial Planet Finder, which consists of two space observatories, one to detect visible light from Earth-like planets and another to probe their chemistry. And the European Space Agency’s Darwin satellite, due for launch in 2015, will scan Earth-like planets for chemical signs of life.

Astronomers are already doing their homework for these missions by simulating how changing seasons on alien worlds would look from Earth. For instance, Darren Williams of Pennsylvania State University in Erie, has simulated the appearance of “snowball Earths” – planets sheathed in ice – as well as ocean-covered “water worlds” with rough or smooth seas, as their starlit crescents wax and wane.

No one knows what the new worlds will look like. But we do know there will be plenty of them. As Jones says, “It’s just going to get bigger and better.” Copernicus would have been proud.